This bird is very appropriately named. It does indeed wag its tail and is similar in this respect to the North American Spotted Sandpiper. The word “pied” means “having 2 or more different colours”.
After Rooks and Jackdaws, Pied Wagtails were the most common bird that I saw in a 2 week trip to Ireland. It is an almost exclusively British bird with just a few nesting in Northern France and Holland.
People sometimes call them Willy Wagtail or Polly Washdish. Long ago people here washed clothing or pots and pans near streams, a favourite habitat for the wagtail, and gave the bird this name.
In Shakespeare’s King Lear, Kent says:
“Spare my grey beard, you wagtail,”
The poet John Clare wrote:
Little trotty wagtail, he went in the rain.
And tittering, tottering sideways he near got straight again.
Wagtails are just as happy on mountain tops as in city centres.
Wagtails nest 2 or 3 times during the summer.
Although almost exclusively insectivorous, some wagtails show a liking for cake crumbs according to a birder that I met.